Semiconductor technology is a basis for much of the modern electronic era including light sources such as light-emitting diodes (LED's). Semiconductor nanocrystals, often referred to as quantum dots, were discovered in the 1980's and have unique properties. Semiconductor nanocrystals are nanometer-sized fragments of the corresponding bulk crystals, and have properties between those of bulk crystals and molecules; they have generated fundamental interest for their promise in developing advanced optical materials. The size-dependent emission is probably the most attractive property of conventional semiconductor nanocrystals. For example, differently sized CdSe nanocrystals can be prepared that emit from blue to red light, with comparatively pure color emissions. These nanocrystal-based emitters can be used for many purposes, such as for solid-state-lighting, solar cells through frequency down conversion, lasers, biomedical tags, and the like.
For any application of semiconductor nanocrystals as emitters, a high photoluminescence (PL) quantum yield (QY) is a basic and well accepted requirement. However, previous work has not recognized that the absorption of the nanocrystals is as important as their quantum yield. In most practical applications, the absorption of the nanocrystals preferably is as high as possible at the excitation wavelength, but as low as possible at the emission wavelengths. Thus, the wavelength of the main absorption band preferably overlaps as little as possible with the wavelength of the emission band; ideally the main absorption band and emission band do not significantly overlap. To obtain semiconductor nanocrystals with both desired emission properties and absorption properties is a challenge. There is a need for nanocrystals with absorption as high as possible at the excitation wavelength and as low as possible at the emission wavelengths.
Types of semiconductor nanocrystals may be classified into plain core nanocrystals and nanocrystalline cores coated with at least one layer of another semiconductor material, commonly referred to as core/shell nanocrystals. The shell layer(s) usually differs from the nanocrystalline core material. Core/shell nanocrystals are likely to be the desired structures when the nanocrystals either undergo complicated chemical treatments, such as in bio-medical applications, or when the nanocrystals are constantly excited, such as diodes and lasers. Core/shell nanocrystals are representative of a number of different complex structured nanocrystals, such as core/shell/shell structured materials, the architectures of which are aimed at providing fine control over the nanocrystal's photophysical properties.
Core/shell semiconductor nanocrystals, in which the core composition differs from the composition of the shell that surrounds the core, are useful for many optical applications. If the band offsets of the core/shell structures are type-I, and the shell semiconductor possesses a higher bandgap than the core material does, then the photo-generated electron and hole inside a nanocrystal will be mostly confined within the core. As used herein, type-I band offsets refers to a core/shell electronic structure wherein the energy level of the conduction band is higher for each consecutive shell compared to the core or to a shell that is closer to the core. The energy level of the valence band is lower for each consecutive shell compared to the core or to a shell that is closer to the core. Conventional core/shell nanocrystals can show high photoluminescence (PL) and electroluminescence efficiencies and can be more stable against photo-oxidation than “plain core” semiconductor nanocrystals comprising a single material, provided that the bandgap of the core semiconductor is smaller than that of the shell semiconductor.
Shells of graded composition, which are multiple monolayers in thickness, are known. See, for example, Liberato Manna, Erik C. Scher, Liang-Shi Li, and A. Paul Alivasatos, “Epitaxial Growth and Photochemical Annealing of Graded CdS/ZnS Shells on Colloidal CdSe Nanorods”, J. Am. Chem. Soc., Vol. 124, No. 24, 7136-7145 (2002) (referred to herein as “Alivasatos”); Renguo Xie, Ute Kolb, Jixue Li, Thomas Basche, and Alf Mews, “Synthesis and Characterization of Highly Luminescent CdSe-Core CdS/Zn 0.5Cd0.5S/ZnS Multishell Nanocrystals”, J. Am. Chem. Soc., Vol. 127, No. 20, 7480-7488 (2005) (referred to herein as “Mews”), both incorporated herein by reference in their entirety. A graded shell composition is useful because the core and shell semiconductors generally have different lattice constants, which can cause significant lattice mismatch. Although graded shell compositions are known, all graded systems are designed with the core as the central concern. The entire shell—including the graded part—is considered as a “protection layer” to boost the emission properties of the nanocrystals. See, for example, WO 2009/014707 to Kazlas, pg. 36. Such a protection layer is thought to prevent the photo-generated charges from being exposed onto the surface of the nanocrystals. The protection layer preferably increases the photoluminescence (PL) quantum yield (QY) by offering a higher chance for the charges to recombine within the core of the nanocrystals and enhance the photostability by eliminating photochemical reactions on the surface of the nanocrystals. Papers published by the Alivisatos group and the Mews group could be considered as typical examples of such core/graded shell/shell nanocrystals.
The “Alivisatos” paper discusses growth of ZnS shell(s) onto CdSe nanorods with CdS as the graded shell between the core and outer ZnS shell to improve the photoluminescence (PL) quantum yield (QY). The photoluminescence (PL) quantum yield (QY) of the resulting CdSe/CdS/ZnS core/shell/shell nanorods is about 10-20%, which is not very high but is significantly improved in comparison to the core nanorods. The “Mews” paper, using a new growth technique (successive-layer-adsorption-and-reaction, SILAR, as described in WO 2004/066361 and U.S. Pat. Pub. No. 2007/0194279, both incorporated herein by reference in their entirety), discloses the growth of CdSe/CdS/Cd0.5Zn0.5S/ZnS core/shell/shell/shell nanocrystals to minimize the lattice mismatch between the CdSe core and the ZnS outer shell, about 12%. The photoluminescence (PL) quantum yield (QY) in “Mews” is as high as 70-85%, but the authors do not focus on optimizing the absorption properties. In other words, the middle CdS and Cd0.5Zn0.5S are introduced as pure “lattice matching” layers. A nanocrystal (CdSe/thick CdS/ZnS core/shell/shell) synthesized by using SILAR, as disclosed in Yongfen Chen, Javier Vela, Han Htoon, Joanna L. Casson, Donald J. Werder, David A. Bussian, Victor I. Klimov, and Jennifer A. Hollingsworth, “‘Giant’ Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking”, J. Am. Chem. Soc., Vol. 130, No. 15, 5026-5027 (2008) (referred to herein as “Chen”), incorporated herein by reference, has quite poor emission properties, but the thick CdS shell offers excellent absorption properties.
There has been much effort directed at using LED's to provide solid state lighting (SSL) devices and to replace traditional light, such as incandescent, fluorescent and halogen lighting, in order to improve energy efficient and increase product lifetimes. However, conventional LED's are restricted in the possible colors they can produce and can have manufacturing and quality control issues.
One approach has been to combine LED technology and with conventional phosphors, which partially absorb the LED light and re-emit light having a longer wavelength. For example, one method of producing white light uses a blue LED wherein a portion of the blue light is used to excite a yellow phosphor, the resulting emission is a combination of blue and yellow light, and thus, white light is produced. However, this approach is also limited in that the type of white light is not readily adjusted; for example, depending on the application, a ‘cold’ white or a ‘warm’ white may be desired and can be difficult to produce using this technology. In order to achieve “warm” white, one could further add red phosphors in the form of mixed phosphors. However, red phosphors show significant absorption of green and yellow emission, which greatly reduce the lumen output of a white LED and related lighting devices.
Incorporation of quantum dot technology into SSL applications has also recently received considerable attention. For example, U.S. Pat. No. 6,501,091 to Bawendi et al., incorporated herein by reference in its entirety, describes producing white light by using an LED source that emits blue light. The blue light passes through a matrix containing quantum dots, which absorb a portion of the blue light and emit green light, the remaining blue light and the green light pass through a matrix containing quantum dots that emit red light, when the light exits this layer it contains the remaining blue light, green light, and red light, and the combination will appear white.
U.S. Pat. No. 6,803,719 to Miller et al., incorporated herein by reference in its entirety, describes a white light-emitting device similar to Bawendi et al., having an LED that emits blue light and a layer containing quantum dots that emit red light formed on the blue LED, followed by a layer containing green light emitting quantum dots. White light can be produced by combining the blue, green, and red light emissions.
The combination of LED chips, phosphors, and nanocrystals has also been explored. For example, U.S. Pat. Pub. No. 2007/0246734 by Lee et al., incorporated herein by reference in its entirety, describes a device for producing white light that includes a green phosphor and a blue phosphor formed on a UV light emitting diode, and wherein a red quantum dot layer is formed on the layer of mixed phosphors.
U.S. Pat. Pub. No. 2005/0135079 by Yin Chu et al., incorporated herein by reference in its entirety, describes the use of nanocrystals and phosphors in the same layer of a white light-emitting device.
U.S. Pat. Pub. No. 2008/0012031 by Jang et al., incorporated herein by reference in its entirety, describes white light-emitting device in which an emission layer includes a combination of a blue LED and a layer having a red luminous body and a layer having a green luminous body. The red and green luminous bodies can be phosphors or quantum dots.
U.S. Pat. No. 7,750,359 to Narendran et al., incorporated herein by reference in its entirety, describes a solid state light emitting device that generates short wavelength light; and quantum dot material and phosphor material that are each irradiated by some of the short wavelength light and wherein emitted light can reportedly have a chromaticity value near the blackbody locus and a color rendering index greater than 80.
However, none of these prior art references disclose a light-emitting material including semiconductor nanocrystals with optimized absorption, emission, and reabsorption properties and having, simultaneously, an emission core surrounded by at least one inner light-absorbing shell, and a protective exterior shell.